That looks like what a specialized modern day Odd Jobs would be.The Paolini pocket rule.
Actually, it would be easy since the smallest hole is 1/8" (0.125") versus 3mm being 0.118". It would just fall through.It must be pretty tricky bashing a 3mm rod through that smallest hole.
In my experience, the best dowels come from first cutting them just a hair oversize with a hollow auger, then popping them through the guide.
Square stock does okay, but it doesn't come out as cleanly as stock that is first cut round.
jtk